The invention relates to Nb-silcide in-situ two-phase composites having improved creep performance. In particular, the invention relates to Nb-silicide based two-phase composites having a certain ratio of niobium (Nb), hafnium (Hf), and titanium (Ti).
Traditionally, turbine components have been often formed from nickel-(Ni) based superalloys materials. These Ni-based superalloys have been used for turbines and turbine components, such as but not limited to, jet engine turbines, land-based turbines, marine-based turbines, and other high temperature turbine environments. The applications of these Ni-based superalloy turbine components may be limited by the high temperatures associated with turbine component operations. Surface temperatures during operation of turbine components can approach temperatures up to and above 1150xc2x0 C., which are approximately 85% of the melting temperatures of the Ni-based superalloy. Therefore, these temperatures can limit the applications of Ni-based superalloys as the high temperatures may adversely influence the Ni-based superalloy""s strength, oxidation resistance, and creep resistance. Further, other intrinsic Ni-based superalloy properties at these high temperatures, such as but not limited to, fracture toughness, high-temperature strength, oxidation resistance, and other mechanical properties, may prevent further applications.
In order to overcome the above-noted deficiencies of the Ni-based superalloys, niobium-(Nb) and molybdenum-(Mo) modified Nb-silicide based composite materials have been investigated for turbine component applications. Niobium has been used to form refractory metal intermetallic composites (hereinafter referred to as xe2x80x9cRMICxe2x80x9d s), which include, but are not limited to, Nb-based refractory metal intermetallic composites. RMICs, such as but not limited to Nb-silicide based composites, possess potentially high operating temperatures, for example, but not limited to, those temperatures encountered in turbine component applications. These RMICs have higher melting temperatures, that Ni-based superalloys, and thus may find beneficial applications in turbine components. For example, some RMICs may have melting temperatures in excess of 1700xc2x0 C., which would be desirable in a turbine component application. See M. R. Jackson et al., xe2x80x9cHigh-Temperature Refractory Metal-Intermetallic Compositesxe2x80x9d, Journal of Materials, January 1996 (pp. 39-44).
RMIC mechanical properties, such as, fracture toughness, high-temperature strength, and oxidation resistance are also enhanced compared to Ni-based superalloys. However, Nb-based refractory metal intermetallic composites may possess poor creep resistance at elevated temperatures, which would be undesirable in turbine components. This creep performance may be due to the existence of an additional hP16 hexagonal phase, which is present in Nb-based composites.
Thus, there is a need for improved high temperature alloys for turbine component applications. Therefore, another need exists to provide a Nb-based material for high temperature applications, in which the Nb-based material can find use in high temperature applications with enhanced creep resistance.
One aspect of the present invention provides a niobium-based silicide two-phase composite that exhibits creep resistance at temperatures equal to or greater than 1150xc2x0 C. The niobium-based silicide composite comprises at least silicon (Si) hafnium (Hf), titanium (Ti), and niobium (Nb). The concentration ratio of Nb:(Hf+Ti) is equal to or greater than about 1.4.
Another aspect of the invention provides a niobium-based silicide two-phase composite that exhibits high temperature creep resistance at temperatures up to about 1200xc2x0 C. The niobium-based silicide composite comprises, in atomic percent, up to about 25% titanium (Ti), silicon (Si) in a range from about 10% to about 22%, hafnium (Hf) hafnium (Hf) in a range from about 2% to about 8%, and a balance niobium (Nb).
A further aspect of the invention provides a method for forming a niobium-based silicide two-phase composite. The composite exhibits creep resistance at elevated temperatures. The method of forming the composite comprises providing silicon (Si), hafnium (Hf), titanium (Ti) and niobium (Nb), and optionally tantalum (Ta), germanium (Ge), iron (Fe), boron (B), molybdenum (Mo), aluminum (Al), chromium (Cr), and tungsten (W). A ratio of Nb:(Hf+Ti) is equal to or greater than about 1.4.